Fighting between rebels and government forces raged near the Syrian capital Damascus on Monday, forcing an inbound commercial jet to turn back while the U.N. said it was withdrawing staff because of deteriorating security conditions.

Lebanese security officials said Jihad Makdissi, a polished Foreign Ministry spokesman known for defending the regime of President Bashar Assad in fluent English, flew from Beirut to London. But it was not immediately clear whether he had defected.

The fighting over the past few weeks in and around Damascus has been the most serious in the capital since July, when rebels captured several neighborhoods before a swift government counteroffensive swept them out. The spike in violence recently is concentrated in the ring of mostly poor suburbs around Damascus but often bleeds into the capital itself as rebels bring their fight closer to Assad's seat of power. Assad's forces have so far repelled major rebel advances on the capital, though their hold may be slipping.

"The security situation has become extremely difficult, including in Damascus," said Radhouane Nouicer, the U.N.'s regional humanitarian coordinator for Syria.

Nouicer said the U.N. was withdrawing most of its international staff from Syria due to security issues, adding that up to one quarter of the 100 international staff working for several U.N. agencies could leave by week's end. There are about 900 more local staff working for the U.N. in Syria, officials said.